1. Field of the investigation
This invention is directed to the bi-directional, multi-channel transmission of light passively across a rotary interface, and more particularly to such transmission in optical sliprings using fiber optic technology.
The ever increasing replacement of electrical data transmission systems with optical data transmission systems necessitates the development of many optical components, functionally equivalent to their electrical counterparts. For example, an electrical slipring is used to transmit electrical signals or power across a rotating interface. Optical rotary joints, such as optical sliprings, enable the transmission of optical signals across such a rotating interface. Presently successful low loss optical rotary joints are limited to single channel transmission devices wherein the optical system occupies the axis of rotation. Attempts have been made for multiple channel devices usually with high or variable light loss, mechanical complexity, large size, signal enhancement, or occupation of the axis of rotation. The present invention enables bi-directional, multi-channel, off-axis transmission across rotary interfaces with comparable light loss in either direction, with low light loss and low light signal variation with rotation, minimal wavelength dependence, and without occupation of the rotation axis of the rotary device to allow use thereof for other purposes.
2. Description of the Prior Art
Optical sliprings can be classified according to the number of transmission channels as either single channel or multi-channel fiber optic sliprings. Such sliprings can be further sub-classified by whether the optical system occupies the rotational axis of the slipring as in an on-axis system, or in an off-axis position; whether the light can travel uni-directionally or bi-directionally with nearly equal efficacy; and whether the slipring can successfully allow light to couple passively, or if the light signals are enhanced electronically, in an active system.
Single channel rotary interfaces have rotary connectors wherein the terminations of two optical fibers confront each other across the rotary interface in the rotation axis of the rotary joint. Light transmission is bi-directional and the optical terminations can either be the bare fiber ends as disclosed in U.S. Pat. Nos. 3,922,063, 4,124,272, 4,373,779, or 4,398,791; or lenses to better focus the light and reduce light loss as disclosed in U.S. Pat. No. 4,373,779 and by the May, 1984 publication of Althouse et al. in The Society of Photo-Optical Instrumentation Engineers - The International Society for Optical Engineering; volume 479, pages 117-120; "A low-loss bi-directional optical rotary joint for fiber-optic applications in fiber optic couplers, connectors, and splice technology".
Yet another variation of an on-axis optical slipring is described in U.S. Pat. No. 4,401,360 wherein two optical receivers are mounted on the rotor and stator axis of rotation, and two optical transmitters are mounted off-centered, but aimed at each of the two receivers. Because the motion of the slipring does not change the orientation of the devices, two optical channels are provided.
Because their geometry lends itself to low light loss, all on-axis optical sliprings can be constructed as passive as well as active optical transmission devices.
Multi-channel optical sliprings are required because of the need for more than one channel enabling transmission of more data with or without wavelength multiplexing, and the option of replacing the complexity and expense of wavelength multiplexing by merely increasing the number of channels. Additionally, multiple channels meet the demand for redundancy lacking in the single channel devices. Moreover, conventional electrical sliprings are often composite devices with the rotational axis occupied by a microwave, hydraulic or pneumatic rotary joint. However, the geometry of a multi-channel optical slipring is often off-axis and permits use of the rotational axis for other purposes. Several different multiple and off-axis optical sliprings have been proposed, which have varied construction and can be grouped according to their basic cptical design as:
1. concentric fiber-bundles
2. waveguides
3. derotating, reflective or transmissive intermediate optical components
4. coaxial, reflective or transmissive intermediate optical components.
A related group are electronic sliprings, which are completely active light devices without optical components, for example as disclosed by Grimes et al. in Instrumentation in the Aerospace Industry - Volume 23 and Advances in Test Measurement - Volume 14, Instrument Society of America, pages 11-19 and entitled "Fiber optic slip rings for rotating test fixture acquisition" (1977).
The simplest, off-axis slipring comprises two opposed annular fiber bundles and increasing the number of such concentric annular bundles radially would make the device multi-channeled. Such devices have been described in U.S. Pat. Nos. 3,922,063, 4,109,997, 4,109,998, 4,124,272, 4,436,367 and 4,492,427 and the publication of Stecyk in "Fiber Optics Data Transmission for an IMU" (1977) in the Final Report for Contract Number IR&D Project No. 94, Report C-4896; The Charles Stark Draper Laboratory, Inc., Cambridge, Mass. The concentric, annular fiber bundle fiber optic sliprings are bi-directional but do have a modulated light loss dependent on the rotational angle, a phenomena described as "eclipsing". The light loss for such geometric slipring construction varies between 4 and 2.5 dB, or a 38% modulation as a percentage of the maximum loss. Increasing the number of fibers in a concentric ring can reduce the modulation as indicated by the publication of Stecyk. It is feasible to terminate the fibers with lenses, and perhaps an array of lenses, to improve light transmission and reduce modulation. Another technique of minimizing the importance of the modulation is to use a digitized signal rather than an analog signal.
A fiber bundle slipring using semicircular, radial fiber arrays is suggested in the aforementioned publication of Grimes et al. and can be made a multi-channel device by stacking along the slipring's rotational axis.
Devices primarily intended for applications other than sliprings, but still requiring a rotary optical coupler, have been described that contain annular fiber bundles in U.S. Pat. Nos. 3,401,232 and 3,411,011.
Waveguide optical sliprings have the common feature of spreading light over 360 degrees of the slipring by illuminating a waveguide. The signal is received by the other rotating member with detectors looking at the waveguide. U.S. Pat. Nos. 4,027,945, 4,109,997, 4,109,998, 4,456,903, 4,466,695, 4,519,673 and 4,525,025 have proposed optical sliprings wherein the circular waveguide is an open, reflective channel. These types of channel waveguide sliprings can be off-axis and capable of having multiple channels by being stacked; however, each channel is essentially uni-directional.
U.S. Pat. Nos. 4,165,913, 4,259,584, 4,277,134, 4,436,367 and Federal Republic of Germany Offenlegunsschrift No. 27 32 806 describe sliprings using solid waveguides which can be transparent rods, tubes, or optical fibers. The light leaks from the solid waveguide into a passive or active receiver because of the bending losses and roughening, splitting, or cutting of the transmitting waveguide or fiber cladding. Light leakage can be assisted by a contacting light-transmitting element such as a roller, slider, or bead of non-wetting liquid. These waveguide sliprings can be off-axis and capable of having multiple channels by being stacked but each channel is generally uni-directional.
One modification would be a transparent ring, and the possible method of building an optical slipring was published by the aforementioned publication of Grimes et al. 1977, who suggested that the fibers of a bundle be radially attached to a plexiglass ring forming one rotating member and that light detectors be used on the other member. Functionally comparable devices have been patented as in U.S. Pat. Nos. 4,107,517, 4,278,323, 4,456,903 and 4,436,367. These light-ring waveguide sliprings are described as both active and passive, off-axis sliprings capable of having multiple channels with possibly bi-directional data transmission. Commercial use of these light-ring fiber optic sliprings is described by Harmer in the 1984 publication in Fibre Optics '84; The Society of Photo-Optical Engineers - The International Society for Optical Engineering; Volume 468, pages 174-185 and having application for optical data links in automobile steering columns. The devices disclosed in U.S. Pat. Nos. 4,107,517, 4,456,903 and 4,436,367 need not employ a continuous 360 degree waveguide; partial light-rings, just large enough to maintain continuity to equally spaced detectors, can be used.
The most efficient optical sliprings are those that rely on an intermediate optical component which moves at half the angular velocity of the slipring with the help of a gear mechanism. The intermediate optical component can be either a reflective or transmissive device and is used to maintain optical continuity of the optical channels. The first described slipring of this type was disclosed in U.S. Pat. Nos. 4,109,997 and 4,109,998 which describe derotating intermediate optical component optical sliprings which used a Dove, Pechan, or other derotating prism as the intermediate optical component.
Devices containing a rotary optical coupler but intended for applications other than sliprings have been described with derotating prisms in U.S. Pat. Nos. 3,350,156, 3,428,812 and 3,997,793. U.S. Pat. No. 4,447,114 describes several disadvantages with sliprings using derotating prisms. Because the devices depend on the refraction of light entering the prisms, the sliprings would be wavelength dependent and thus could be built to operate only at one wavelength of light. Because of the long ray paths in the sliprings, divergence of the light becomes an important factor, especially for the Pechan prism. The lens arrays of these sliprings would also have to be aligned accurately and each lens separated at some distance from its neighbor to prevent light straying from one channel to another, resulting in crosstalk. To overcome these limitations, there is disclosed in U.S. Pat. No. 4,447,114 a derotating optical slipring wherein the intermediate optical component is a glass sphere bisected by a thin mirror. The intermediate optical component is located on the rotation axis and can have shapes other than a sphere. This patent also suggests a mirror at the junction of the bases of two cones in one configuration. The optical element can also be placed off center, but such a geometry produces a complex mechanical design with two, perpendicular axes of rotation.
Both U.S. Pat. Nos. 4,447,114 and 4,460,242 disclose derotating optical sliprings that use a transmissive intermediate optical component comprising an annular or circular array of optical fibers. A continuous optical channel is maintained by an unspecified arrangement of the fibers within the intermediate optical component. U.S. Pat. No. 4,258,976 describes a derotating optical slipring which uses a derotation plate as the intermediate optical component. The derotation plate is a packed array of optical fibers inverted through the center of the plate in their passage from the entrance face to the exit face of the plate. These derotating intermediate optical component optical sliprings can have multiple, bi-directional channels and can be either active or passive.
Multi-channel optical sliprings achieved with lenses or mirrors arranged coaxially to the axis of rotation have been described by U.S. Pat. No. 4,519,670 and by Harstead et al. (1986) in the Summaries of Technical Papers presented at the Optical Fiber Communication Conference, Feb. 24-26, 1986 sponsored by the Institute of Electronic Engineers & the Optical Society of America, pages 70-72 in a paper entitled "Low-loss multifiber optical rotary joint". Generally the optical path of a channel from one rotating member to another is maintained by the use of an intermediate lens, mirror or combination of lenses and mirrors which maintain the same relative position with respect to the rotation axis of the slipring during rotation. As in the case of the derotating intermediate optical component optical sliprings, this is accomplished by gearing; however, in the case of the coaxial optical sliprings the various optical components must rotate synchronously to maintain the optical paths.
The technical challenge in building a multi-channel, off-axis optical slipring is having the incoming light available for transmission over a 360 degree rotation, transmitting the light across the gap with minimum loss, dispersion, and modulation, then collecting sufficient light on the receiving side to conduct it along the fiber. A multiple channel optical slipring can be either an active or passive device, but the ideal slipring would be a passive device not having the limitations imposed by electronic components such as slower data rates or electromagnetic interferences.
Additionally, such an optical slipring would have an unoccupied rotational axis and would be bi-directional, transmitting the same, high percentage of light in both directions regardless of the rotational angle. Such an optical slipring should also efficiently transmit a range of light wavelengths simultaneously. The proposed optical slipring would not be size-dependent, allowing for any number of separate channels in any diameter. The ideal optical slipring would also be immune from dirt, dust, and moisture. However, all of the prior art fiber optical sliprings have at least one or more of the aforementioned limitations, or lack one or more of the aforementioned advantageous features and attributes.